add an agda script
update the hevea version number

The hevea download web site is frustrating: tar files for one version
disappear as soon as a newer one is available, and there is no URL
available for accessing the most recent one.  So, if the download fails,
look at the web site and bump the version number here.
remove a redundant line from with.sh
add compilation of "checker" to coq-builder; add DEVEL_DAN option